Recovery of hydrocarbons from a subterranean formation by a combination of in situ combustion and water flood



United States Patent RECOVERY OF HYDROCARBONS FROM A SUB- TERRANEANFORMATION BY A COMBINA- TION OF IN SITU COMBUSTION AND WATER FLOODCharles L. Bauer, New York, N.Y., assignor to Texaco Inc., New York,N.Y., a corporation of Delaware No Drawing. Filed June 19, 1968, Ser.No. 738,118

Int. Cl. E21b 43/24 U.S. Cl. 166251 8 Claims ABSTRACT OF THE DISCLOSUREImproved recovery of hydrocarbons from subterranean hydrocarbon-bearingformations is effected by a combination of in situ combustion and waterinjection. In situ combustion is caused to occur in random pockets in asubterranean formation, followed by water injection thereinto toscavenge the generated heat as part of a water drive and thereby produceadditional hydrocarbons contained therein. During injection of acombustion-support ing gas into the formation, effluent gases aremonitored at output wells until the carbon dioxide content of theeffluent gases exceeds 4% and the percent of oxygen accounted for hasincreased to about 50% to indicate when in situ spontaneous ignition ofhydrocarbons has occurred.

FIELD OF THE INVENTION This invention relates to an improved process forthe recovery of hydrocarbons from subterranean hydrocarbon-bearingformations. More particularly, this invention relates to a method whichexploits favorable recovery conditions produced by conducting an in situcombustion operation prior to water injection.

DESCRIPTION OF THE PRIOR ART One of the newer developments for therecovery of hydrocarbons from subterranean hydrocarbon-bearingformations has been by in situ combustion operation. In the conventionalmethod of applying this process, the hydrocarbon-bearing formation ispenetrated by an injection well and one or more offset wells. Air isprovided to the formation via the injection well, and combustion of thehydrocarbons adjacent the well bore in the forma tion is initiated byany one of the many accepted means, whereby a combustion zone is createdin the form of a narrow cylindrical zone immediately surrounding thewell bore. The air injection is continued until the combustion zone isdriven radially from the injection well toward one or more offset wells,and the resulting thermal drive forces the displaced hydrocarbons in theformation toward one or more offset wells by direct or reverse drive.

More recent developments have created a thermal drive based upon acombination comprising subjecting the formation surrounding the wellbore to in situ combustion by conventional means and conducting thecombustion front radially outward more or less uniformly into thehydrocarbon-bearing formation to allow the generated heat to beaccumulated in the formation, whereupon the air injection is terminatedcausing the combustion to cease. The air injection well is thenconverted to a water injection well, making it possible to flood theformation with water. The water injected into the formation absorbs theresidual heat in the formation behind the front, and because of thenature of the heat capacities and thermal conductivities of water, itbecomes an efficient transfer medium of this heat to the rest of theformation. The thermal drive is therein moved a further distanceoutwardly resulting in displacing and forcing additional hydrocarbonstoward the offset production wells.

3,520,363 Patented July 14, 1 970 One of the difficulties in utilizingthe combination of the in situ combustion and water injection has beenthe relatively confined areas of the formations in which the subsequentwater injection step can scavenge effectively and thus utilize the heatwhich has been generated in the formation. That transfer by conductionfrom the source of heat to a portion of the reservoir remote from thesource of heat is relatively slow. For example, it has been estimatedthat the temperature of the formation may be increased only -l00 F. fora distance up to 30 feet from the zone of combustion by conduction alonein about 2 years.

Accordingly, it is an object of the present invention to overcome thislimitation by providing a method whereby heat is generated moreeffectively and advantageously throughout the reservoir, which is thenmore effectively utilized by a water injection step.

SUMMARY This invention comprises subjecting the subterraneanhydrocarbon-bearing formation to an in situ combustion wherein the insitu combustion is caused to occur spontaneously and randomly throughoutthe hydrocarbonbearing formation, which is then followed by a waterinjection step which takes advantage of the favorable recoveryconditions created thereby.

DESCRIPTION OF THE PREFERRED EMBODIMENT More specifically, it has beenfound that an in situ combustion which has been spontaneously initiated,randomly in a plurality of sites within a hydrocarbon-bearing formation,causes heat to be generated more effectively within thehydrocarbon-bearing formation which can then be utilized in a subsequentwater injection procedure.

The method comprises initiating combustion in a formation spontaneouslyby injecting a combustion-supporting gas until evidence that combustionhas occurred to a significant extent is shown by both an increase in thecarbon dioxide content and an increase in the percent of oxygenaccounted for in the efiiuent gases from monitored offset wells, atwhich time the injection of the combustion-supporting gas is terminated,and water injection is undertaken. The percent of oxygen accounted formay be defined as the ratio in percent of the atomic oxygen appearing inthe efiluent gases to the atomic oxygen in the injection gas.

Experience has shown that injection of a combustionsupporting gas, i.e.one which contains oxygen, through a formation containing hydrocarbonswhich are susceptible to auto'oxidation, can effect spontaneous ignitionof the hydrocarbons contained therein. The sequence leading toinitiating an in situ combustion by this means involves auto-oxidationfollowed by spontaneous ignition of the crude. Auto-oxidation, whichresults from the slow oxidation of hydrocarbons 'by the oxygen in thegas, occurs initially at a relatively low rate with an accompanying slowrelease of exothermic heat. With continued injection of the gas into theformation, however, autooxidation occurs to a significant extent andeffects on appreciable rise in the temperature of the formation,resulting in spontaneous ignition of the hydrocarbons.

The manifestation of the reaction sequence and the rate at which it isoccurring can be obtained by continuously monitoring and analyzing theefiiuent gases from offset wells. In particular, it is desirable todetermine the carbon dioxide content and the percent of oxygen accountedfor in the effluent gases. The determination of the carbon dioxidecontent may by made by any desired means, e.g. by employing an Orsatapparatus, and the percent of oxygen accounted for can be determinedfrom appropriate calculations using stoichiometric principles.

When a hydrocarbon undergoes auto-oxidation, it has been observed thatit is accompanied by an appearance of carbon dioxide and by a low valueof percent of oxygen accounted for in the effluent gases with theaccompanying rise in temperature within the formation, due to theexothermic oxidative reactions, the carbon dioxide content of theeffluent gases will continue to rise and the percent of oxygen accountedfor will increase. As the temperature of the formation rises, theignition temperature of the hydrocarbons is approached and spontaneousignition becomes imminent. The time at which this point in the reactionsequence is reached is approximately indicated when the carbon dioxidecontent in the effluent gases exceeds about 4%, and the percent ofoxygen accounted for exceeds about 50%, although there is an indicationthat combustion is occurring when the percent oxygen accounted for isabout 30%.

Experience has shown also that spontaneous ignition of a formation bythis reaction sequence may occur randomly within the formation, and notnecessarily in the immediate vicinity of the injection well bore, butrather in a plurality of sites or pockets wherever the concentration ofthe hydrocarbons, the concentration of the oxygen, and the formationcharacteristics produce optimum conditions for its occurrence. Thisrandomness is evidenced by the differences in carbon dioxide content andthe percent of oxygen accounted for observed in the effluent gases fromthe offset wells located throughout the field. Generally, when the sitesat which spontaneous ignition is occurring are located in relativelyclose proximity to those offset wells, from which the effluent gasesindicate an increasing carbon dioxide content and an increasing percentof oxygen accounted for, stimulation of fluid production is observedalso. The location of these sites is affected by such factors as thecharacter of the hydrocarbon, the oxygen content of the injected gases,the rate of injection, the formation pressure, and temperature.

In most formations wherein ignition has been accomplished spontaneously,about 8 to 10 weeks are required, although there are instances in whichlonger periods of time have been required.

The tendency of a hydrocarbon to undergo auto-oxidation leading tospontaneous ignition, and the rate at which it will occur can bemeasured in the laboratory by means of experiments in which a sample ofthe hydrocarbon is contacted with an oxygen-bearing gas. Measurements ofoxidation rates can also be determined in the laboartory using samplesof the representative hydrocarbon-bearing formation. Such measurementswill give an indication of how long a time will be required before theinjection of the combustion-supporting gas will result in spontaneousignition.

The susceptibility of hydrocarbons to undergo spontaneous ignitionvaries, and appears to be greater in lower API gravity crudes, i.e.crudes whose API gravities are less than As will become apparent, thepresent invention is particularly applicable to field-wide operations,utilizing a plurality of wells, which are not necessarily in anydefinite pattern. However, it may be applied to a specific area within afield. The offset wells included in the operation of this method shouldbe adaptable as monitor wells and production wells, and convertible towater injection wells for subsequent water flooding.

In accordance with the invention, there is provided a method whichinvolves as its essential steps passing a combustion supporting gasthrough a formation containing hydrocarbons to effect spontaneousignition of the hydrocarbons and thereby generate heat in the formation,monitoring the composition of the effluent gas to determine the extentto which spontaneous ignition has occurred within the formation, andthereafter passing an aqueous medium through the formation to scavengethe 4 in situ heat and drive the hydrocarbons to a production well.

Illustrative of the efficacy of this invention was its application to aproducing field in Trinidad, of about 40 acres, in which thehydrocarbon-bearing formation contained an l8.4 API crude. The formationwas traversed by one injection well and 18 offset wells, more or less,randomly spaced, and up to distances of about 1200 feet from theinjection well. In this field, the air injection step was conducted forabout 11 months until an estimated 4.5 x10 B.t.u.s of heat had beengenerated in situ. Air injection was then terminated and thereupon seawater was injected into the formation via the former air injection well.For the period of 22 months during which time approximately 75,000barrels of sea water were injected, it was estimated that an additional51,200 barrels of oil were produced with approximately 62% of theproduction occurring at wells located from 800 to 1200 feet away fromthe site of the original air injection well.

A procedure for the application of this invention to ahydrocarbon-bearing formation which is susceptible to spontaneousignition would comprise the following steps:

(a) Inject a combustion-supporting gas at ambient temperatures or aboveinto the formation via an injection well or wells, while monitoringeflluent gases from a plurality of offset wells. The efiluent gaseswould be analyzed for carbon dioxide content and the percent of oxygenaccounted for would be determined by appropriate calculations. Theperiod of gas injection would continue until evidence-that spontaneousignition had occurred within the formation was shown by at least 4% ofcarbon dioxide appearing in the effluent gases of offset Wells togetherwith an increase in the percent of oxygen accounted for;

(b) Continue injection of the combustion-supporting gas for a sufficientperiod of time to allow the in situ combustion thus initiated to heat asubstantial portion of the formation to temperatures above the ignitiontemperatures of the hydrocarbons in the formation, i.e. above 450-500 F.The length of this period can be determined from calculations of theheat generated in situ, based on the air injection rates and theeffluent gas compositions;

(c) Terminate injection of the combustion-supporting (d) Commence waterinjection. Either the former gas injection well or one or more of theoffset wells may be used for Water injection. The selection of offsetwells for conversion to water injection wells will depend upon theestimated combustion pattern and sites of combustion, the fluidmovements, and the location of the prospective water injection wells inrelation to the production wells. Water injection may be timed tocontrol the amount of heat generated by the in situ combustion and thethermal energy left in the reservoir; and

(e) Continue water injection until sufficient amounts have been injectedto have scavenged effectively the generated heat from in situcombustion. The amount of water can be calculated from the thermalproperties of the formation and the fluids, and the in situ generatedheat. The mechanics of the heat transfer are not known entirely, but itis postulated to involve successive vaporization and condensation of thewater. Thus, the in situ heat is conserved and further utilized withinthe formation by transferringheat from the hot rock formation to thehydrocarbons to make the hydrocarbons more mobile and thereby moreeasily produced. The water used may be fresh water, brine, or watercontaining carbon dioxide.

It will be apparent from the foregoing description that the method issubject to other modifications without departing from the scope of theinvention as defined in the following claims.

I claim:

1. A method of recovering hydrocarbons from a subterraneanhydrocarbon-bearing formation penetrated by at least one injection welland offset wells, comprising the steps of:

(a) injecting into said formation through said injec tion well acombustion-supporting gas having oxygen to cause spontaneous ignition ofsaid hydrocarbons in a random plurality of sites in said formation,

(b) monitoring efiluent gases from said offset wells and determiningcarbon dioxide content and percent of oxygen accounted for in saidefiluent gases until the said carbon dioxide content of said effluentgases exceeds 4% and said percent of oxygen accounted for has increasedto about 50%,

(c) continuing injection of said combustion-supporting gas into saidformation through said injection well until a substantial portion ofsaid formation in said random sites has undergone in situ combustion asdetermined by step (b); and thereafter terminating said injection ofsaid combustion-supporting gas,

((1) thereupon, injecting into said formation an aqueous medium throughsaid injection well, and producing said hydrocarbons from said offsetwells, and

(e) continuing injecting said aqueous medium into said injection well toscavenge effectively said generated heat in said formation.

2. The method of claim 1 wherein the injected combustion-supporting gasis air.

3. The method of claim 1 wherein the injected aqueous medium is selectedfrom the group consisting of fresh water, brine, water saturated withcarbon dioxide, or mixtures thereof.

4. In the method of claim 3, injecting said aqueous medium intospecified offset wells in close proximity to offset wells whereat thecarbon dioxide content is greater than about 4% and said percent ofoxygen accounted for is greater than about 5. In the method of claim 1,producing said hydrocarbons from specified offset wells wherein thecarbon dioxide content in the effluent gases is greater than about 4%,and said percent of oxygen accounted for is greater than about 50%. r

6. In the method of claim 1, producing from specified offset Wellsshowing increased fluid production and initiating the injection of saidaqueous medium into said offset wells in close proximity to saidspecified offset wells.

7. In the method of claim 1, said wells being included in any givenpattern in a producing field, the steps of consecutively changing anoffset well which has signs of high carbon dioxide content in saideffluent gas to a production well and converting said adjacent offsetwells to said aqueous medium injection wells.

8. In a method of claim 1, the steps of changing the function of saidoffset wells from monitoring wells to production wells when the carbondioxide is greater than about 4% thereat.

References Cited UNITED STATES PATENTS 3,036,632 5/1962 Koch et al.166256 3,064,728 11/1962 Gould 166261 3,072,185 1/1963 Bond et al.166261 3,221,809 12/1965 Walton 166263 STEPHEN I. NOVOSAD, PrimaryExaminer U.S. Cl. X.R.

